The issue of how to effectively transport large amounts of data across fluctuating radio channels is a challenging hurdle. For instance, high quality multimedia (MM) streams that use highly compressed data, such as the video compression standard MPEG-4, are very susceptible to low quality fluctuating channel conditions. There are two fundamentally different approaches to deal with fluctuating radio channels, one at the video processing layer, and the other at the physical layer. Due to the extremely high variation in channel quality, inversion of the fading effects of the fluctuating channel with adaptive power control is not desirable, as the power level could have to be varied dramatically with the maximum power level at least several tens of times higher than the average power level. Due to this challenge at the physical layer, much effort has been focused on the design of a video processing layer that treats the fluctuating channel fading condition and the resultant errors as a given and subsequently focuses on how to adapt to and recover from it.
However, advancements in adaptive antenna array (AAA) technologies such as BLAST (Bell Laboratories Layered Space Time), which uses multiple spatial sub-channels within a single frequency channel, has made it possible to address channel fluctuations at the physical layer since dramatic changes in transmitting power are no longer necessary to inverse channel fading. This is because multiple antennas and spatial sub-channels provide diversity, which can “smooth out” the aggregate channel fluctuation across different transmission periods. This is conceptually similar to statistical multiplexing. Certain simulation results have shown that a 2×2 antenna matrix system can witness a 45% drop in the standard deviation of the average fading level among all sub-channels in each transmission period as compared with the single antenna case, and an 8×8 system can witness a 75% drop in the deviation. AAA systems are described in more detail in U.S. Pat. No. 6,097,771 issued to Foschini and U.S. Pat. No. 6,317,466 issued to Foschini et al., both of which are fully incorporated herein by reference.
The use of multiple antennas does introduce another dimension of variation referred to as diversity, or the variation in fading levels among the various spatial sub-channels. The equal allocation of resources, including both power and bit rate, to each transmitting antenna as implemented in most AAA systems, is inefficient. In fact, simulation results show that a significant amount of power is wasted to maintain a low target bit error rate (BERtarget) and probability of outage (P(outage)).
There are generally two reasons for the inefficiency. First, each sub-channel can experience vastly different fading conditions, so in order to compensate for the worst fading scenario to maintain a low BERtarget and P(outage), extra power needs to be allocated to ensure that the signal-to-noise ratio (SNR) of each received symbol in every sub-channels is high enough for accurate detection. Safe-guarding the power level for the worst scenario results in significant waste in the other sub-channels having less severe fading levels.
Second, the above is also applicable to sub-channel bit rate allocation. In most typical resource allocation schemes, each sub-channel transmits at the same bit rate. When a specific sub-channel suffers from severe fading and, hence, a potentially high BER, the resources allocated to that sub-channel are essentially wasted unless transmitting power is increased. But such a power increase, on the other hand, is wasted on the sub-channels having a sufficient SNR.
Accordingly, improved communication systems are needed that can efficiently allocate resources to multiple antennas within a wireless communication environment.